The problem of stabilizing the rotor rotational speed characteristic for such vibration motors is settled in certain well-known designs by using a speed stabilization circuit.
Known in the art, for example, is a vibration motor with a side-driven rotor provided with a number of equispaced holes equidistant from the center. Pressed against the side surface of the rotor is a concentrator of longitudinal vibrations adjoining working, shielding and feed-back piezoelectric cells. The working piezoelectric cell is connected with the output of a high frequency oscillator, whose inputs are connected one to an adjustable d-c power supply source and the other to a frequency separator; the shielding piezoelectric cell electrode is grounded; the feed-back piezoelectric electrode is connected with the said d-c power supply source via a frequency separator and a frequency detector.
Such a vibration motor operates in the following way.
Upon excitation of the working piezoelectric cell mechanical vibrations emerge in the cell. The frequency separator receives high-frequency oscillations of the piezoelectric cell in a form amplitude-modulated by a low-frequency signal generated due to periodic variation of the load imposed to the concentrator. The high-frequency oscillations are supplied to the working piezoelectric cell. The low-frequency oscillations are fed to the frequency detector which converts any change in the oscillation frequency into a voltage change. The voltage change is applied to the regulating element of the d-c power supply source to obtain an output voltage proportional to the rotor speed change.
Such a vibration motor is distinguished by a high stability of the rotor speed, but its power is comparatively low. Also known in the art is a vibration motor comprising a concentrator of torsional vibrations and a rotor held firm against the end face of the concentrator and provided with marks equally spaced along a circumference and intended for operation in conjunction with a rotor speed pickup. The latter is connected with a piezoelectric cell unit, built into the concentrator of torsional vibrations, via a frequency detector, a d-c power supply source and a high-frequency electric oscillator connected to each other in series.
Provision of a greater contact area between the concentrator and the rotor makes it possible to obtain a higher torque on the shaft of such a vibration motor. But in such vibration motors it is impossible to make use of a prior art rotor with holes to periodically vary the load imposed to the concentrator, because the concentrator driving stage is fashioned as a sleeve in distinction to a single rod-like configuration peculiar to the prototype.
It is a common practice to make use of a photoelectric or magnetic-flux gate transmitter as a rotor speed pickup in the vibration motors of this kind. Such transmitters are expensive and not simple in design. Moreover, they complicate the vibration motor construction, which is undesirable for use of such complicated vibration motors in electric playback devices particularly.